layout: Add initial support for the ::marker pseudo-element (#36317)

This change adds support for the `::marker` pseudo-element and ensure
that
markers are cached into the box tree. This is only initial support,
there are a few
things missing such as animations, transitions, and support the
`content` CSS
property.

Testing: There are WPT tests for this change.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout_2020/dom.rs

@@ -34,6 +34,7 @@ pub struct InnerDOMLayoutData {
     pub(super) self_box: ArcRefCell<Option<LayoutBox>>,
     pub(super) pseudo_before_box: ArcRefCell<Option<LayoutBox>>,
     pub(super) pseudo_after_box: ArcRefCell<Option<LayoutBox>>,
+    pub(super) pseudo_marker_box: ArcRefCell<Option<LayoutBox>>,
 }
 
 /// A box that is stored in one of the `DOMLayoutData` slots.
@@ -222,6 +223,7 @@ where
         let cell = match pseudo_element_type {
             PseudoElement::Before => &data.pseudo_before_box,
             PseudoElement::After => &data.pseudo_after_box,
+            PseudoElement::Marker => &data.pseudo_marker_box,
             _ => unreachable!(
                 "Asked for box slot for unsupported pseudo-element: {:?}",
                 pseudo_element_type
@@ -235,6 +237,7 @@ where
         let cell = match pseudo_element_type {
             PseudoElement::Before => &data.pseudo_before_box,
             PseudoElement::After => &data.pseudo_after_box,
+            PseudoElement::Marker => &data.pseudo_marker_box,
             _ => unreachable!(
                 "Asked for box slot for unsupported pseudo-element: {:?}",
                 pseudo_element_type
@@ -248,6 +251,7 @@ where
         *data.self_box.borrow_mut() = None;
         *data.pseudo_before_box.borrow_mut() = None;
         *data.pseudo_after_box.borrow_mut() = None;
+        *data.pseudo_marker_box.borrow_mut() = None;
         // Stylo already takes care of removing all layout data
         // for DOM descendants of elements with `display: none`.
     }

components/layout_2020/dom_traversal.rs

@@ -236,6 +236,10 @@ fn traverse_element<'dom, Node>(
 ) where
     Node: NodeExt<'dom>,
 {
+    // Clear any existing pseudo-element box slot, because markers are not handled like
+    // `::before`` and `::after`. They are processed during box tree creation.
+    element.unset_pseudo_element_box(PseudoElement::Marker);
+
     let replaced = ReplacedContents::for_element(element, context);
     let style = element.style(context);
     match Display::from(style.get_box().display) {

components/layout_2020/flow/construct.rs

@@ -393,7 +393,7 @@ where
                 is_list_item: false,
             },
             NonReplacedContents::OfPseudoElement(contents).into(),
-            BoxSlot::dummy(),
+            info.node.pseudo_element_box_slot(PseudoElement::Marker),
         );
     }
 
@@ -405,7 +405,7 @@ where
     ) {
         self.block_level_boxes.push(BlockLevelJob {
             info: info.clone(),
-            box_slot: BoxSlot::dummy(),
+            box_slot: info.node.pseudo_element_box_slot(PseudoElement::Marker),
             kind: BlockLevelCreator::OutsideMarker {
                 contents,
                 list_item_style,

components/layout_2020/lists.rs

@@ -19,11 +19,7 @@ pub(crate) fn make_marker<'dom, Node>(
 where
     Node: NodeExt<'dom>,
 {
-    // TODO: use `PseudoElement::Marker` when we add it.
-    let marker_info = info.pseudo(
-        context,
-        style::selector_parser::PseudoElement::ServoLegacyText,
-    )?;
+    let marker_info = info.pseudo(context, style::selector_parser::PseudoElement::Marker)?;
     let style = &marker_info.style;
     let list_style = style.get_list();
 

components/layout_2020/query.rs

@@ -131,7 +131,7 @@ pub fn process_resolved_style_request<'dom>(
         None => layout_element,
     };
 
-    let style = &*layout_element.resolved_style();
+    let style = &*layout_element.style(context);
     let longhand_id = match *property {
         PropertyId::NonCustom(id) => match id.longhand_or_shorthand() {
             Ok(longhand_id) => longhand_id,
@@ -1122,7 +1122,7 @@ where
     let element = node.as_element().unwrap();
     let parent_style = if node.is_connected() {
         if element.has_data() {
-            node.to_threadsafe().as_element().unwrap().resolved_style()
+            node.to_threadsafe().as_element().unwrap().style(context)
         } else {
             let mut tlc = ThreadLocalStyleContext::new();
             let mut context = StyleContext {

components/script/dom/window.rs

@@ -1262,6 +1262,7 @@ impl WindowMethods<crate::DomTypeHolder> for Window {
                 Some(PseudoElement::After)
             },
             Some(ref pseudo) if pseudo == "::selection" => Some(PseudoElement::Selection),
+            Some(ref pseudo) if pseudo == "::marker" => Some(PseudoElement::Marker),
             Some(ref pseudo) if pseudo.starts_with(':') => {
                 // Step 3.2: If type is failure, or is a ::slotted() or ::part()
                 // pseudo-element, let obj be null.

components/shared/script_layout/wrapper_traits.rs

@@ -363,22 +363,6 @@ pub trait ThreadSafeLayoutElement<'dom>:
             .clone()
     }
 
-    /// Returns the already resolved style of the node.
-    ///
-    /// This differs from `style(ctx)` in that if the pseudo-element has not yet
-    /// been computed it would panic.
-    ///
-    /// This should be used just for querying layout, or when we know the
-    /// element style is precomputed, not from general layout itself.
-    #[inline]
-    fn resolved_style(&self) -> Arc<ComputedValues> {
-        let data = self.style_data();
-        match self.pseudo_element() {
-            None => data.styles.primary().clone(),
-            Some(pseudo_element) => data.styles.pseudos.get(&pseudo_element).unwrap().clone(),
-        }
-    }
-
     fn is_shadow_host(&self) -> bool;
 
     /// Returns whether this node is a body element of an html element root